So I read that you randomly unlock characters by beating different cups in Grand Prix Mode, and that you cannot unlock another character by replaying the cup.

However, I noticed that there are 14 unlockable characters, but only eight cups. How does that work? Does it sorta reset once you've beaten all the cups? Do some characters come bundled together (like Rosalina and Baby Rosalina, or Wendy and Roy)? Or are there six characters you just can't unlock? (in which case if I get the game, I'll have to hope Iggy's one of the eight I get)

Well I unlocked all the characters already. Ironically the first character I unlocked was Iggy, the main one I was after (so I did all other races as him, except two where I decided to try out Ludwig and Roy, who was the second last I unlocked, last being Morton)